1. Field of the Invention
This invention relates in general to a protective circuit for a load and in particular to a protective circuit for a load which is coupled to a push-pull amplifier supplied with power voltages from separate voltage sources.
2. Description of the Prior Art
Conventional amplifiers utilized in audio systems not using output transformers have a single-ended push-pull amplifier utilizing pairs of transistors in the last stage which are connected in series to a power source. An output signal is supplied to a speaker from the amplifier through a capacitor of relatively large capacitance. Such capacitors between the output of the amplifier and the speaker prevent DC bias voltage from being applied to the speaker. However, when the frequency of an audio signal is low, the signal may be substantially attenuated by the coupling capacitor and thus the low frequency components of the signal may be substantially attenuated and will not be reproduced by the speaker. It has been proposed in the art to avoid this disadvantage of the coupling capacitor by using a two-source system in which the transistors of the last stage are connected in series to each other and are respectively supplied with positive and negative voltages. This allows the connection point of the transistors to be held at substantially ground potential and the speaker is directly connected between the output terminals of the transistors and the ground. Thus, in such an arrangement, both sides of the speaker are at the same DC potential, so that the speaker is driven directly with the output of the amplifier without receiving any DC bias voltage and is capable of producing signals of very low frequency.
However, with such an amplifier there may be a fear that an abnormal voltage such as a DC voltage or the like appears at the output terminal of the amplifier to damage the speaker. To avoid this, it is desirable to provide a detecting circuit which may detect a DC potential between the final stage amplifier and the speaker. A protective circuit may be provided at the input terminal of the speaker which is operated with the output of the detecting circuit. By way of example, a DC voltage is provided by a resistor and a capacitor connected between the output transistor and the speaker and a relay or SCR (semi-conductor controlled rectifier) is driven by the DC voltage.
In general, however, troubles with amplifiers are frequently caused by class A amplifying stage failures and therefore both the output stage or class B amplifying stage and the drive stage or class A amplifying stage of the amplifier must be considered when designing protection circuits. The speaker must be protected positively even if the class A amplifying stage becomes inoperative.